1. Field of the Invention
The invention relates to a display device that makes a display with pixels arranged in rows and columns, and more specifically to a display device that has memory circuits each of which stores an image signal for a respective one of the pixels and that controls the writing of image signals into the pixels from the memory circuits in accordance with control signals.
2. Description of the Related Art
In recent years, digital information equipment, such as personal computers, has achieved a remarkably high level of performance and their information processing capabilities have advanced by leaps and bounds. Correspondingly, display devices for displaying the results of information processing have also attained a significant increase in display capacity.
However, conventionally used CRT (cathode ray tube) display devices have increased their size as the displays have increased in screen size and display capacity. In particular, an increase in the depth, weight and power dissipation becomes a problem. To solve the problem, flat panel displays, particularly liquid crystal displays (LCDs), have been used. However, the LCDs, which are very difficult to manufacture, have not been up to the level of the CRTs in screen size and resolution.
With the LCDs, digital signals are used for display signal connection with digital information equipment. Thus, in comparison with CRTs in which analog signals are used, the number of signal lines to be connected increases significantly. In addition, image signals need to be transferred at high rates. The high-rate transmission of image signals each of many bits results in the generation of electromagnetic noise and an increase in signal transmission power.
Moreover, an increase in the amount of image data to be transmitted results in an increase in the time required to update the display contents on a screen. The updating of the entire screen is slow in comparison with the updating of only a small area of the screen. Thus, motion-image display degradation will occur due to the slowdown of the movement of moving objects on screen.
In recent years, multi-window systems have been increasingly used in which a plurality of images are displayed on screen. A window image which has been hidden below other images after being displayed once must be retransferred via a video memory as with motion images. Thus, power dissipation increases every time images are switched from one to another. In addition, a time delay involved in switching images increases.
For the LCDs, under these circumstances, the power dissipation has been reduced by making their driving voltages or driving frequencies lower. As a structure that allows a further decrease in power dissipation, a structure that is equipped with a memory for each pixel has been proposed (Japanese Unexamined Patent Publication No. 58-196582 or No. 3-77922).
According to this technique, for a still image, once an image signal has been sent to each pixel, it can be driven constantly by the signal retained by its associated memory. Therefore, in theory, the power is dissipated only in reversing polarity and hence the power dissipation is approaching zero.
In recent years, however, multimedia systems have been increasingly used, so that the display of motion images is in increasing demand. In motion images, pixel information changes at a high rate. Even if each pixel has a memory, therefore, it is required that the memory be rewritten into at a high frequency. Such high-rate rewriting of pixels will result in a considerable increase in power dissipation as with the conventional LCDs.
With the pixel-memory-equipped LCD described previously, image data are held in each pixel memory and the memory contents are used to display the pixel. For still image display, this helps reduce the driving frequency and the static power dissipation. For motion image display, however, it is naturally required to increase the driving frequency, which will result in an increase in the whole power dissipation.
In particular, the display of motion images has become essential with the recent spread of multimedia. The LCDs are often used in portable equipment, such as portable personal computers, hand-held terminals, portable TV sets, cellular phones, electronic notebooks, game machines, etc. Therefore, the power dissipation problem with LCDs is one of important problems to be solved.